The invention relates to a high average power pockels cell for use generally with laser beam propagation applications.
Pockels cells are utilized in the laser art to alter the polarization state of a laser beam which is shined through a crystal. Birefringent crystals can alter the polarization state of laser beams that are shined through them. For example, an initially linearly polarized laser beam may emerge from the crystal elliptically polarized, or it may emerge linearly polarized, but rotated by 90 degrees. In a pockels cell, a voltage is applied across the crystal, creating an electric field E.sub.v inside the crystal, which alters the birefringence of the crystal through the electro-optic effect. The polarization state of the emerging laser beam can then be varied by varying the voltage. Pockels cells are used as polarization switches in beam steering applications and as amplitude modulators. The crystal has special directions related to the directions of the planes of the atoms in the crystal lattice. The orientation of the crystal is specified by the directions of its three mutually perpendicular crystal axes which are called the C, X and Y axes, and which are tied to the crystal lattice. The nature of the pockels effect is that the relative directions of the electric field (E.sub.v), the crystal axes (C,X,Y) and the direction of propagation (K) of the laser beam are all quite important. There are many standard designs for pockels cells that are suitable for low average power laser beams.
However, prior art low average power designs are generally not suitable for high average power laser beams because the crystal absorbs some of the laser light, which consequently results in increased temperature. Since the crystal is heated in its middle by the laser beam and cooled on the edges by its supports, the crystal gets hotter in the middle than on the edges. The temperature variation gives rise to various effects which also change the laser's polarization state independently of the electric field E.sub.v. These effects cause the part of the emerging laser beam that propagated near the center of the crystal to have a different polarization state than the part that propagated near the edge of the crystal. Consequently, such variation makes the device relatively useless for most applications. The temperature dependent effects that dominate the degradation are different for the different types of common pockels cell designs in the prior art. Two of the more common effects are thermally induced strains and the temperature dependence of natural birefringence.
Strain is a distortion of the crystal which can be characterized in two classes. In order to understand the two classes, consider an imaginary square inside the crystal with its sides parallel to two of the crystal axes, say X and Y. A tensile-compressive strain is a distortion of the square into a rectangle. The other type of strain is a shear strain which is a distortion of the square into a diamond. A crystal can distort because the hot parts expand more than the cold parts. The strains can alter the polarization state of the laser beam through the strain induced birefringence effect.
Another important effect is the temperature dependence of natural birefringence. Natural birefringence determines the polarization state of the emerging laser beam in the absence of applied voltage or strain. The voltage and strain induced effects add on top of this natural birefringence.
Various approaches have been taken in the prior art for dealing with the problems that arise from the temperature variations that occur in high average power applications. All of these approaches utilize a two-pass pockels cell in which the laser beam goes through two crystals or through the same crystal twice. There are common low average power designs that use both one and two passes, but which, as described above, are unsuitable for high average power application purposes.
The prior art high average power designs may be summarized in the following manner. Upon entering a first crystal, the laser beam splits into a fast and a slow polarization in which the polarization directions and the speed of propagation are dictated by the natural birefringence, the strains and the voltage applied. In between the crystals, the two polarizations are interchanged. The laser beam then goes through a second crystal which is identical to the first crystal in all ways other than the applied voltage. The net result is that all effects on the final polarization state other than the voltage induced effect (and therefore all thermal effects) cancel.
The prior art approach, in general, is not to make the thermal effects in one crystal small, but to make the thermal effects in two separate crystals identical so that they can cancel one another. In principle, these devices null out both of the effects of thermal induced strains and temperature dependence of natural birefringence.